The present invention relates to a convertiplane.
As is known, convertiplanes are hybrid tilt-rotor aircraft combining the advantages of fixed-wing, turbo-propeller aircraft and helicopters.
A fixed-wing aircraft (airplane) is maintained airborne by the lift produced by the wing. Sufficient lift to sustain the aircraft, however, can only be achieved over and above a given minimum speed, which, in turn, can only be achieved by accelerating the aircraft on a runway of appropriate length. This obviously also applies to commuter turbo-propeller airplanes. Once airborne, a turbo-propeller airplane is capable of reaching and maintaining a cruising speed of about 500 km/h and a typical altitude of 7500 meters enabling it to fly over most cloud formations and atmospheric disturbance. The continual increase in air traffic, however, will eventually result in smallxe2x80x94e.g. 15-30-passengerxe2x80x94aircraft being excluded from long-distance routes, by occupying traffic windows which could be used more profitably by larger aircraft. Fixed-wing aircraft also pose environmental problems, particularly as regards acoustic pollution of residential areas around airports, which are necessarily covered at low altitude.
A helicopter is maintained airborne by the lift provided by the rotor, which is rotated relative to the air by the engines with no need for horizontal speed, so that a helicopter can take off and land vertically on small surfaces not much larger than itself. For a helicopter to acquire horizontal speed, part of its lift must be directed forwards, which is done by slightly tilting the rotor disk. Limitations inherent in the aerodynamics of the rotor, however, limit the maximum speed of a helicopter to just over 300 km/h, with a normal cruising altitude of, at best, 3000-3500 meters and a smaller range than an airplane with the same load and fuel capacity. A fundamental characteristic typical of helicopters is their ability to hover, which accounts for their being used almost exclusively in civil and military search, rescue or surveillance applications calling for direct intervention in unequipped areas.
To combine the speed, range and efficiency of a fixed-wing, turbo-propeller aircraft with the hovering and vertical takeoff and landing ability of a helicopter, convertible aircraft or convertiplanes have been developed. These are characterized by an external structure similar to that of a turbo-propeller airplane, but have two rotors, on the ends of the wing, which can be tilted upwards to sustain the craft in helicopter mode, and forwards for thrust in airplane mode, thus enabling the craft to take off and land in small spaces, in exactly the same way as a helicopter, and to cruise at high speed and altitude like an airplane. The advantages, with respect to a conventional helicopter, of such a combination therefore lie in almost twice the cruising speed (500 as opposed to 300 km/h); substantially twice the range for a given load and fuel capacity, which obviously makes the craft more profitable; and over twice the cruising altitude, so that the craft is unaffected by weather conditions (clouds, turbulence) over most of the flight. The advantages with respect to a conventional airplane, on the other hand, lie in the ability of the craft to take off and land in small spaces, and to hover.
Convertiplanes, however, are highly complex aircraft which are extremely difficult to control. Which, together with the high cost involved, is why they have so far only be used on a small scale, and substantially only for military applications.
At present, substantially two convertiplane configurations are known: tilt-rotor and tilt-wing.
In the case of the first, each engine-rotor assembly can be rotated with respect to the relative half-wing to direct the thrust of the rotor upwards when taking off, landing or hovering, or forwards for cruising. This type of configuration has several drawbacks.
In particular, the downward load on the wing caused by the wash of the rotors seriously impairs hovering performance. The resisting force (in opposition to the thrust of the rotors) may reach 10-14% of the aircraft""s weight, which is a significant percentage when compared with the pay load to total aircraft weight ratio of about 20%. To minimize the phenomenon, the wing is provided with movable surfaces of a much larger area and a much wider range of movement than a conventional fixed-wing aircraft, which greatly increases the mechanical complexity of the wing.
Moreover, during conversion, the flow induced by the rotor on the wing combined with the flying speed causes the wing to operate with a very large angle of attack with respect to relative wind, which in turn causes stalling problems accompanied by vibration, oscillatory loads and high resistance. Stalling also reduces the so-called conversion corridor, i.e. the range of permissible rotor positions and aircraft speeds during conversion from helicopter mode to airplane mode and vice versa.
Another problem posed by interaction of the rotors and the fixed wing of tilt-rotor convertiplanes is the difficulty in achieving stable autorotation in helicopter mode.
In the so-called tilt-wing configuration, the position of the rotors is adjusted by rotating the whole wing-propulsion system assembly to reduce interaction of the rotor wash and the wing in helicopter mode. On the other hand, conversion from helicopter to airplane mode and vice versa is extremely critical, which is why this configuration has found very little practical application.
It is an object of the present invention to provide a convertiplane designed to eliminate the aforementioned drawbacks typically associated with known types.
According to the present invention, there is provided a convertiplane comprising:
a fuselage;
a wing having a central portion fixed to said fuselage, and two half-wings projecting from said fuselage;
two rotors carried by respective supporting structures at the ends of said half-wings, and having respective shafts;
two engines fixed to respective said half-wings by respective supporting means;
transmission means connecting each of said rotors to both said engines; and
means for changing the position of said rotors between a helicopter mode wherein said shafts of said rotors are substantially vertical, and an airplane mode wherein said shafts of said rotors are substantially horizontal;
characterized in that said means for changing the position of said rotors comprise a beam element extending the full length of the wing, rotating about a respective axis, and rigidly interconnecting said supporting structures of said rotors; and first actuating means for rotating said beam element about said axis;
said half-wings comprising respective fixed portions adjacent to said fuselage, and respective movable portions rotating about said axis of said beam element;
said convertiplane comprising second actuating means for rotating said movable portions with respect to said fixed portions and to said beam element.
In a preferred embodiment of the present invention, said supporting means of said engines are also connected rigidly to said beam element.